Your Desktop Board might have had a BIOS version that's lower (earlier) than 0484 installed on your particular board. You will need BIOS version 0484 or higher in order to use that E7500 on your DG33BU Desktop Board.
Pheraps I think this is speedStep technology
when cpu are in idle state fequency multiplier change from 11x (266Mhz x 11 = 2.93GHz) to 6x (266Mhz x 6 = 1.6GHz) to save energy.
check this switch of cpu multiplier with cpu-z and open a hevy program.
I update my BIOS and now its working fine